Stock material or miscellaneous articles – Self-sustaining carbon mass or layer with impregnant or...
Reexamination Certificate
2001-03-15
2003-11-11
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Self-sustaining carbon mass or layer with impregnant or...
C252S512000, C252S513000
Reexamination Certificate
active
06645629
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a conductive antioxidant paint and a graphite electrode, and more particularly, to a conductive antioxidant paint especially suitable as an antioxidant material for a graphite electrode used in arc furnaces such as electric steel-making furnaces, and a graphite electrode coated with such a conductive antioxidant paint.
Hitherto, artificial graphite electrodes have been used in conventional arc furnaces such as electric steel-making furnaces. Upon the use, the graphite electrodes have been exposed to extremely severe conditions such as large electric current, high temperature, splashes of molten steel or the like. In particular, since ultrahigh-temperature arcs are generated at a tip end of the electrode, the graphite electrode has been exposed to a temperature as high as about 400 to about 3,000° C. As a result, the graphite electrode is readily oxidized and consumed by an oxidative gas entering through openings of the furnace.
In the electric steel-making furnaces, costs for the electrode used therein is very high, so that the consumed electrode leads to large economical loss. Upon the oxidation, 50 to 70% by weight of a consumed portion of the electrode is due to oxidative attack from a side surface of the electrode, and the arc itself has a less influence on the wear-out of the electrode. The wear-out of the electrode by oxidation becomes more remarkable at the position closer to the tip end thereof, so that the electrode is tapered toward the tip end, resulting in accelerated wear-out of the electrode in the longitudinal direction. Accordingly, if the oxidative attack from the side surface of the electrode is sufficiently prevented, the wear-out of the electrode can be effectively reduced with a large economical merits.
Conventionally, various methods have been proposed in order to inhibit the oxidation of the electrode. For example, there are known a method of applying to the electrode an antioxidant paint composed of a matrix containing a glazing material (frit) having a melting point of not more than 1,000° C., and refractory aggregates (Japanese Patent Application Laid-Open (KOKAI) No. 48-72211); a method of forming a non-conductive antioxidant paint layer on the surface of graphite electrode (Japanese Patent Application Laid-Open (KOKAI) No. 59-51499); a method of applying onto the electrode, a paint prepared by dispersing alumina or silica fine particles in a colloid solution containing silica ultrafine particles (Japanese Patent Application Laid-Open (KOKAT) No. 3-45583); or the like.
However, since any of these conventional paints is non-conductive, it is required that a chuck portion of the electrode is released from being coated therewith in order to ensure the current flow to the electrode. For this reason, there arise problems such as complicated coating processes, insufficient oxidation-resistant property at the uncoated portion of electrode or the like. In particular, in the method described in Japanese Patent Application Laid-Open (KOKAI) No. 48-72211, when the frit is softened at a temperature of not more than 1,000° C, the coating film undergoes shrinkage, so that film defects such as peeling-off, penetration or ruptures tend to be caused. In order to eliminate these film defects, there has been proposed a method of repeating the coating works until the thickness of the obtained coating film after melting the frit becomes as large as about 1 mm. However, in such a case, the working efficiency has been considerably deteriorated.
Further, in order to solve the above problems, there have been proposed conductive antioxidant paints of the type which can be coated even on the chuck portion of electrode by imparting a conductivity to the obtained coating film (Japanese Patent Application Laid-Open (KOKAI) Nos. 7-268248, 7-268249 and 7-268250). More specifically, Japanese Patent Application Laid-Open (KOKAI) No. 7-268248 describes a conductive antioxidant material containing a refractory aggregate, a colloidal binder and carbon black without any glass frits; Japanese Patent Application Laid-Open (KOKAI) No. 7-268249 describes a conductive antioxidant material containing a refractory aggregate, a binder and graphitized carbon black; and Japanese Patent Application Laid-Open (KOKAI) No. 7-268250 describes a conductive antioxidant material containing a refractory aggregate, a binder, carbon black and a polymer emulsion. The refractory aggregate includes oxides such as silica, alumina, titania or zirconia, and the binder includes inorganic colloid.
However, these conventional conductive antioxidant paints generate a large amount of hydrogen gas during the storage or transportation, resulting in poor storage stability thereof and breakage of a container therefor. For this reason, a long-period storage and a long-distance transportation of these paints are difficult.
In addition, in the case where the conventional conductive antioxidant paints are coated on the electrode, the obtained coating film suffers from pinholes by the hydrogen gas generated therefrom, so that the electrode tends to be burned from the pinholes. Thus, the conventional conductive antioxidant paints are deteriorated in antioxidant property when exposed to a temperature as high as not less than 1,000 ° C. As a result, the electrode is not only deteriorated in heat resistance and oxidation resistance, but also more likely to be consumed by the oxidation.
Meanwhile, it is known that the temperature within the arc furnace reaches not less than 1,000° C., thereby causing the following problems concerning the conventional conductive antioxidant paints. That is, the oxidation-resistant effect of the above paints is exhibited by a glass-like coating film formed by heat-melting film-forming components of the paints at a temperature of about 800° C. However, when the paints are heated to a temperature as high as not less than 1,000° C., the obtained glass-like coating film is unsuitably lowered in viscosity and fallen off from the surface of the electrode, resulting in a discontinuous glass-like coating film. Such discontinuous glass-like coating film fails to prevent the contact between oxygen and the electrode, thereby causing defects such as burning-in of the electrode, e.g., cissing thereon.
As a result of the present inventors' earnest studies to solve the above problems, it has been found that (1) the generation of hydrogen gas is caused when the paint has a high pH value, and is due to by the reaction between a trace amount of alkali components and impurities of the metal compound; (2) the viscosity of the glass-like coating film is lowered by melt-penetration of the alkali components into the coating film; or (3) the generation of hydrogen gas is caused by the existence of specific elements in the paint. The present invention has been attained based on the above finding.
More specifically, it has been found that the deterioration in viscosity of the glass-like coating film is due to such a phenomenon that alkali metals such as sodium and potassium or compounds thereof such as alkali metal oxides, and alkali earth metals such as calcium and magnesium or compounds thereof such as alkali earth metal oxides are melt-penetrated into the glass-like coating film after burning of carbon, and that the deteriorated viscosity of the glass-like coating film causes falling-off or separation of the coating film from the electrode, thereby adversely affecting the antioxidant effect of the electrode. In addition, it has been found that the above alkali components tend to react with the impurities of a metal compound to generate a hydrogen gas, thereby considerably deteriorating a storage stability of the paint, and that when hydrogen gas is generated during drying step of the coating film, pinholes are formed therein, so that the electrode tends to be burned from the pinholes.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved conductive antioxidant paint which is inhibited from generating a hydrogen gas during the
Fukushima Tatsuo
Hisashi Hideyuki
Kawabata Issei
Conlin, Esq. David G.
Edwards & Angell LLP
Jones Deborah
Koppikar Vivek
Mikuni Color Ltd.
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